Systematic Electronic and Structural Studies of 9-Carbene-9-Borafluorene Monoanions and Transformations into Luminescent Boron Spirocycles.

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (-) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound , which contains [B]-K(THF) interactions, is compared to charge-separated -, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the B NMR spectra of charge-separated borafluorene monoanions show distinct low-field signatures compared to . Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] () is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable RBO spirocycles are formed ( and , respectively). Remarkably, compounds and display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.